Conversion in chemical reactions is limited by the equilibrium position of the reaction. If the chemical equilibrium in a synthesis reaction is only partly to the side of the products, a one-stage reaction regime leads only to partial conversion. If, by contrast, the reaction products are removed continuously from the reactor, continuous conversion of reactants to products takes place in the reactor.
Many of the chemical reactions of economic relevance are equilibrium-limited. In chemical synthesis, these are, for example, the production of methanol from hydrogen, carbon monoxide and/or carbon dioxide, for the production of ammonia from hydrogen and nitrogen, called the Haber-Bosch process. These reactions nowadays take place in heterogeneous catalyzed fixed bed reactors or slurry reactors. The reactants here are only partly converted in a single pass through the reactor. Thereafter, the reactant/product mixture is drawn off, the reaction products are typically separated off and unconverted reactants are recirculated to the reactor inlet.
The recirculation of volumes of gas that are large in some cases leads to high apparatus complexity. The pressure drop in the recirculation has to be compensated for by means of a compression unit. This is usually operated at high temperatures and leads to high costs and high energy expenditure. Moreover, the recirculation results in accumulation of inert gases and extraneous gases in the circuit, which has adverse effects on the reaction regime.